In many business applications, color documents have become essential as a component of communication. Color facilitates the sharing of knowledge and ideas. Companies involved in the development of color output devices continue to look for ways to improve the total image quality of such devices. One aspect that affects the perception of image quality is the consistent production of a quality image by a printer—from one document to the next, from one day to another, from one week to the next, etc. Users are accustomed to printers and copiers that produce high quality color and grayscale output. Users further expect to be able to reproduce a color image with consistent quality on any compatible output device, including another device within an organization, a device at home or elsewhere. Hence, there remains a commercial need for efficiently maintaining print color predictability and consistency.
Recently filed patent applications including U.S. Pat. No. 6,975,949 for a FULL WIDTH ARRAY SCANNING SPECTROPHOTOMETER by L. K. Mestha et al., awarded Dec. 13, 2005, and U.S. application Ser. No. 11/016,952 for a FULL WIDTH ARRAY MECHANICALLY TUNABLE SPECTROPHOTOMETER, by L. K. Mestha, filed Dec. 20, 2004, cover the use of two different types of full-width array (FWA) spectrophotometers to sense a full page. Both are also hereby incorporated by reference for their teachings. Similarly, U.S. application Ser. No. 10/248,387 for SYSTEMS AND METHODS FOR OBTAINING A SPATIAL COLOR PROFILE, AND CALIBRATING A MARKING SYSTEM, by L. K. Mestha et al., filed on Jan. 15, 2003, and also hereby incorporated by reference, shows an approach that utilizes a specially designed test pattern containing numerous patches, some of which are arranged side by side along the slow scan direction (y-direction) to obtain the gray tonal reproduction curves (TRCs).
Color printing customers not only require that their documents (images) are accurately reproduced in color, but also demand that the outputs do not vary over time. Color consistency, over time is often achieved or maintained by calibration. Most of the existing printer calibration methods are template based—meaning that they require the printer to print a set of test templates. The actual outputs are then measured and compared with the desired outputs to generate the error signal that drives the calibration process. The template-based approach is typically “offline” and “discrete” in nature. Thus, such an approach requires stopping the normal printing process. While this might not be particularly disturbing for short runs, where the calibration might be scheduled between print jobs, it is certainly an inconvenience and an impact to efficiency when a long run print job needs to be interrupted for calibration. Also, off-line calibration is a “discrete” event, meaning there is typically a significant time interval between two consecutive calibrations, where appreciable color error may accumulate during the period between calibrations.
Disclosed herein is a system and method to provide improved color consistency performance by calibration performed continuously in an on-line fashion for each page, yet using customer (print job) images. The disclosed method includes at least two phases, a training phase and a calibration phase. While the calibration is performed continuously during printing, the prior training phase is undertaken offline where the total color drift is modeled as a linear combination of a few Eigen functions. The resulting information is then used to dynamically control color drift of any image from page to page.
Instead of printing test templates, the disclosed system and method extracts calibration information from a plurality of image pages printed when inline sensors or spectrophotometers (full width/partial width/low cost light emitting diode (LCLED)) are used as measurement devices in the printing system. The calibration phase is “real time” and “continuous” and can use colors off the incidental patch areas of the print image directly. Also disclosed is a low-rank approximation method employed to increase the color consistency performance of the system when the measurements are made from incidental patch areas in the printed image. Advantages of the disclosed method include, among others, convenience to the customer (calibration is “embedded” in the printing process and undetected by users), and accuracy in calibration (calibration is performed continuously so that any color drift is compensated in a timely fashion and color variation is significantly reduced.)
A main difference between the template-based calibration and the image based approach is the number of measurements. In the template-based calibration since one can print as many patches as required and measured offline, it has a full control of the measuring points. The test templates are carefully designed to ensure they are capable of capturing and measuring errors in any part of the color space. In contrast, in the image based method, calibration information is extracted from the printed customer images, with randomly created measurement points at various locations in the image. Such measuring points are thus image dependent and the calibration system will have very limited control. A major challenge for the image based approach is to derive color shifts over the entire color space from the available, limited set of measurements, appearing at random times while running a print production job. Accordingly, the disclosed systems and methods are directed to efficiently calibrating color print engines for uninterrupted (continuous) service using inline sensors.
Disclosed in embodiments herein is a method for image based control, comprising: a training process, wherein a total color drift is modeled; and a calibration process performed continuously during completion of a printing job.
Also disclosed in embodiments herein is a method for controlling performance of an image output device, comprising: characterizing the variability of the image output device in terms of a model; periodically collecting data relating to a current state of the image output device; and applying the data to the model to determine an adjustment to be made to the image output device.
Further disclosed in embodiments herein is a system for image based color calibration, comprising: an image output device for rendering printed color documents in response to digital image data an initial configuration as processed by a digital front end processor operatively connected to the system; a memory for storing information characterizing the performance of the image output device; a color sensor suitable for detecting a color of an image deposited on the printed color documents and producing an output indicative of the color; and a calibration processor for receiving the output indicative of the color and, in conjunction with the information characterizing the performance of the image output device, producing a modified configuration for subsequent use in rendering the printed color documents.